Bradycardia Pacing Induced VT/VF

Abstract & Commentary

By John P. DiMarco, MD, PhD Professor of Medicine, Division of Cardiology, University of Virginia, Charlottesville. Dr. DiMarco is a consultant for Novartis, and does research for Medtronic and Guidant.

Source: Sweeney MO. Bradycardia pacing-induced short-long-short sequences at the onset of ventricular tachyarrhythmias. A possible mechanism of proarrhythmia. J Am Coll Cardiol 2007;50:614-622.

Sweeney et al analyzed the initiation of ventricular arrhythmias on stored event electrograms from 1055 patients enrolled in 2 large ICD trials. (PainFREE Rx II and EnTrust). In these trials, the programming for ventricular tachycardia (VT) and ventricular fibrillation (VF) detection was standardized by protocol. In the VF zone, detection required 18 of 24 R-R intervals that were < 320 ms. In the VT zone, the cycle length range for detection was either 320 to 360 ms or < 400 ms. for 16 consecutive intervals. Pre-onset electrogram storage was turned on in all patients. Dual chamber ICDs were used in 75% of the PainFREE Rx II study patients and in 100% of the EnTrust trial patients. In the latter group, 92% were programmed to the Managed Ventricular Pacing (MVP) mode. MVP functions basically as an AAI/R mode that monitors ventricular conduction during atrial based pacing and switches to DDD/R pacing if AV block occurs.

All stored episodes of VT/VF were analyzed. Episodes were then classified into 4 groups: 1) non-pacing associated, 2) pacing associated, 3) pacing permitted, and 4) pacing facilitated. VT/VF episodes associated with "short-long-short (S-L-S) sequences were called "pacing permitted" if the pacing mode passively allowed pauses longer than the specified lower rate interval. Episodes were called "pacing facilitated" if the pacing mode initiated or terminated a S-L-S sequence at the onset of VT or VF with a ventricular paced beat.

In the database used for this report, a total of 1,356 spontaneous episodes of VT/VF with pre-onset electrograms were available for analysis. Non-pacing associated and pacing associated onset patterns were seen in 44.0% and 29.8% of all episodes. Pacing permitted or pacing facilitated S-L-S sequences were associated 26.2% of the events with the frequency related to the basic programmed pacing mode. Pacing permitted initiation was more common with the VVI/R and MVP modes since these modes try to avoid unnecessary ventricular pacing. Pacing facilitated initiation was more common with the DDD/R mode. The VVI/R mode was associated with longer pauses at VT/VF initiation since the lower rate limit in this mode was typically set at 40 beats per minute. Pacing facilitated S-L-S initiation was observed in 2.6% of patients with the MVP mode, 5.2% of the DDD/R pacing, and 3.3% of the VVI/R patients. Most pacing facilitated S-L-S episodes were monomorphic VT as opposed to VF. A small number of patients had only pacing permitted or pacing facilitated episodes of VT/VF.

The authors conclude that ventricular pacing can result in S-L-S sequences that may initiate episodes of VT/VF in ICD patients. Further refinement in bradycardia pacing algorithms are needed to reduce the frequency of this complication.


Sudden changes in ventricular cycle length and short-long-short sequences leading to abnormal patterns of ventricular repolarization can cause ventricular tachycardia and ventricular fibrillation. Bradycardia pacing arrhythmias can produce such cycle length changes and new pacing modes designed to reduce the frequency of right ventricular pacing may unintentionally make sudden cycle length change more common. In this paper, Sweeney et al demonstrate that all forms of ventricular pacing can result in VT/VF initiation. Different patterns were observed with the MVP, VVI/R, and DDD/R modes but all modes could be associated with episodes. In some patients, only pacing facilitated VT/VF episodes were seen. This problem has been under recognized in the past. Unfortunately many ICD's do not have pre-onset electrogram storage programmed on, so data about the initiation of individual VT/VF episodes is often not available to the electrophysiologist caring for the patient. This paper should make us aware that the programmed bradycardia pacing mode may actually be contributing to the frequency of VT/VF episodes and supports approaches designed to minimizing unnecessary ventricular pacing.